ik heb de oplossing al gevonden, voor mensen die het ook nodig hebben:
Another fix if the above fails:
1. Run nVidia X Server settings which you can find in the Application menu.
2. Select X Server Display configuration and chose the save to x configuration file option. This will build file xorg.conf. Close Nvidia X Server settings.
3. sudo gedit /etc/X11/xorg.conf in terminal.
4. Find this section:
Section "Device"
Identifier "Device0"
Driver "nvidia"
VendorName "NVIDIA Corporation"
BoardName "GeForce GT 540M"
EndSection
5. And change it to something like this:
Section "Device"
Identifier "Device0"
Driver "nvidia"
VendorName "NVIDIA Corporation"
BoardName "GeForce GT 540M"
Option "RegistryDwords" "EnableBrightnessControl=1"
EndSection
6 Reboot.